The invention relates to a method of providing a contactless on-line measurement of the surface structure or thickness of a material or a layer on a material in plate or strip form as well as to a measuring arrangement.
Texture parameters are an essential quality feature of metal surfaces. The most well known is the representation of the surface roughness, the extensive data on a measured roughness profile section in practice usually being reduced to just a few characteristic variables, such as for example the peak-to-valley height Ra and the average peak-to-valley height Rz.
In roughness measurement, it is generally not necessary to scan and measure the entire surface uninterruptedly. It is enough if a sufficiently large number of measurements permit an adequate statistical finding. In the case of a fast-moving material, considerable difficulties are involved in detecting the roughness by means of an on-line measurement. The traditional stylus method is not in any way capable of producing satisfactory results at speeds greater than 20 m/min. Therefore, such on-line measurements have long been carried out on an optical basis.
For instance, a roughness measuring instrument which is used in the field of the metal-processing industry as an on-line measuring instrument for continuous process control is described in the company publication of Sick GmbH Optik-Elektronik, Munich "SORM Sick optisches Rauheitsmess system" (SORM Sick optical roughness measuring system). With the aid of a semiconductor laser, the measuring instrument generates a fine laser measuring beam on the material surface. The light spot diameter is about 10 .mu.m. The surface structure causes the impinging light to be reflected in a certain spatial direction, depending on the flank angle of the surface facet. The principal direction of the reflection is detected by a laser diode receiver array and converted into an angle-dependent analog signal. This analog signal is digitized and the surface level profile calculated on the basis of the flank angle data and the advancement information. Consequently, a profile characteristic which is approximately comparable with a mechanical measurement is obtained as raw data. Subsequently, this raw data is evaluated in the usual way and the standardized roughness variables, such as Ra, Rz, are determined and output.
In the case of this measurement, the intensity distribution reflected from the surface falls, in the form of a scattering lobe via a number of lenses, onto the assigned photoreceiver array arranged annularly around the laser light spot. The photoreceivers forming this photoreceiver array are interconnected in the form of a PSD sensor (position sensitive detector). The direction of the point of concentration of the scattering lobe is measured directly at each measuring point. Consequently, the flank steepness of the respective surface facet is determined optically at each surface measuring point. Upon movement of the surface to be measured under the measuring head, the surface profile is calculated by back-integration. The roughness measuring area is, for Ra 0.05, up to 2.5 .mu.m, and the material speed lies in the range of 0.3 to 30 m/sec.
This measuring instrument is very complex since the optical measuring head contains, in addition to a semiconductor laser, the receiver array, an autofocussing device for correcting the optical measuring head, and a signal preprocessing means. Furthermore, an evaluation computer with two floppy disk drives, an interface for a plurality of optical measuring heads and a central processing unit are required.
A surface measuring system RM 600, with which surface structures between 0.02 .mu.m and 600 .mu.m can be detected quickly and contactlessly is described in the company publication "Profilmess platz RM 600 2-D" (Profile measuring station RM 600 2-D) of Rodenstock, Munich. The measuring system essentially comprises three components, namely an optical distance sensor, a linear or X/Y advancing unit and a control computer with color screen or monitor, printer, and software package. The key component of the system is an optical sensor which registers changes in distance by means of a laser focus of 1 or 2 .mu.m in diameter. In measurement, the measured object is moved uniformly past the sensor. The measured values thereby occurring form a level profile which can be output by the control computer as a graphic and which can be measured. By suitable filtering, findings on individual surface parameters, such as roughness, waviness etc. are possible.
The optical distance sensor operates with an infrared laser, the beam of which is focussed on the surface of the measured object. Depending on the type of sensor, a light spot of 1 or 2 .mu.m in diameter is produced on the measured surface and imaged in the sensor onto a focus detector. If the distance from the measured surface changes, the detector supplies a control signal for the automatic focus correcting means. A plunger coil system then displaces the objective until the laser beam is again focussed exactly on the surface of the measured object.
Since the focus distance is constant, the movements of the objective correspond exactly to the level variation of the measured surface. The respective position of the objective is detected by an inductive sensor and supplies a measuring signal which is independent of the reflection characteristics of the measured surface. This surface measuring system is not suitable for the measurement of fast-running materials.
Apart from the surface structure of materials, their surface condition is also a quality feature which can be used for controlling the processing of such materials. The surface condition determines, inter alia, the reflectivity or the diffuse reflectance value of the surface for incident radiation in the ultraviolet or infrared range. For example, the thickness of the aluminum oxide layer on the surface of an aluminum strip or an aluminum plate is the determining parameter for the reflection of incident infrared radiation on the material surface.
This also applies in the same way to layers of different composition, such as for example layers of light-sensitive substances, with which aluminum strips or plates are coated during the production of printing plates. As the thickness of the coating of such materials on the surface increases, the surface loses reflectivity, and the reflected infrared or thermal radiation decreases.
If, for example, transparent materials are concerned, such as plastics films, as the thickness of these materials increases, their reflectivity on the surface decreases.